1. Field of the Invention
The present invention relates to a passive optical network for communication between an optical line terminal and a plurality of subscriber' optical network units by use of code division multiplexing (CDM) and time division multiplexing (TDM).
2. Description of the Related Art
Passive optical network (PON) systems that provide bidirectional communication between a central node and a plurality of subscribers' units are often considered for use as optical access network systems. The main advantage of a PON system is that by sharing an optical transmission path among a plurality of subscribers, it can reduce system equipment costs.
Typical of the PON systems now in use are gigabit-PON (G-PON) systems, which operate at wavelengths from 1250 to 1500 nanometers. Different wavelengths are allocated to the upstream traffic from the subscribers' units to the central node and the downstream traffic from the central node to the subscribers' units, as described by McGregor et al. in ‘Implementation of a TDM Passive Optical Network for Subscriber Loop Applications’, J. Lightwave Technology, Vol. 7, No. 11, pp. 1752-1758 November 1989.
PON systems that employ CDM are also attracting attention. Examples of such CDM-PON systems are described by Tamai et al. in ‘Jisedai hikari akusesu shisutemu COF-PON no kenkyu kaihatsu’ (Research and development of COF-PON: a next-generation optical access system), Oki Technical Review, Issue 210, Vol. 74, No. 2, April 2007, by Kashima et al. in ‘Ko-QoS maruchi media hikari haishin shisutemu no kenkyu kaihatsu—COF transhiba’ (Research and development of high-QoS multimedia optical distribution system—COF transceiver) Oki Technical Review, Issue 200, Vol. 71, No. 4, October 2004, by Sasase in ‘Hikari shisutemu ni okeru hikari fugo bunkatsu tagen setsuzoku gijutsu’ (Optical Code Division Multiple Access Techniques in Optical Communication Systems) TELECOMFRONTIER, November 2004, and by Matsumoto in Japanese Patent Application Publication No. 2003-317026.
These CDM-PON systems enable simultaneous multiplexing of a plurality of signals, and offer a large communication capacity while conserving communication resources such as wavelengths and time slots. A CDM-PON system of the synchronous CDM type disclosed by Kashima in Japanese Patent No. 3913139, for example, which carries out synchronization on each channel, has the further advantage of being able to extract signals on an arbitrary channel from the CDM signal.
CDM-PON optical access enables longer-reach transmission than TDM optical access. In CDM-PON optical access the same wavelength can be used to carry signals in both directions, so CDM-PON systems can easily be combined with wavelength division multiplexing (WDM) optical access.
A future requirement for optical access services will be to combine data communication with video communication. Present WDM-PON systems use cable television transmission technology to transmit and receive analog video signals in optical form. Future PON systems will need to find a way to combine digital video transmission with data communication.
One issue to be addressed is the much greater information content of a video signal, as compared with a data communication signal. Another issue is the real-time nature of video transmission. If video signals are transmitted in digital form, accordingly, a large guaranteed bandwidth, that is, a high assured bit rate, must be provided in real time.
In current WDM video communication systems in which an analog video data signal is transmitted and received in optical signal form as described above, if the signal is transmitted over a long distance, the receiving equipment must include an optical amplifier, but optical amplifier installation costs are high. In a long-reach PON system, in which each subscriber's receiving unit requires an optical amplifier, a large number of optical amplifiers are necessary, significantly increasing the system cost.
When TDM access and analog optical signal transmission and reception are implemented in a G-PON system, the sensitivity of the photodetector and the signal-to-noise ratio of the optical signal must be sufficiently high. The receiving unit therefore requires a wavelength filter that can provide a strong degree of isolation of the received signal. Such filters are costly, which is another factor that increases the system cost.
Present G-PON systems do not provide enough bandwidth to accommodate additional digital video signals in the same time slots as used for telephony and other purposes. If digital video transmission is desired, it is necessary to add a parallel G-PON system with a different pair of wavelengths, one for upstream transmission and one for downstream transmission. Besides requiring additional wavelength resources, this requires additional wavelength filters, greatly increasing the system complexity and cost.
A series of studies by the present inventor led to recognition that a PON communication system configured by combining a CDM-PON communication system with a conventional G-PON communication system is capable of transmitting a large amount of digital data, including digital video signals, without changing the configuration of the conventional G-PON system and without greatly increasing the use of wavelength resources.